My back foot keeps slipping in my animation?
You need to bring correction to the feet positions. What you can do is:


Put your 3D view in Right Orthographic. In the Dopesheet / Action Editor mode, load the walk animation called "stairs". Enable the Only Include Channels Relating To Selected Object option so that you'll only see the selected bone keyframes.





Look when the first slipping happens, it's between frame 23 and 30.





So what you need to do is correct the foot position at frame 30 so that if stays at the same position as frame 23. Delete the current frame 30 keyframe. As your root bone is moving, you can't just copy paste the keyframes of 23, you need to visually position the foot at frame 30 and create new keyframe (i > LocRot). Also make sure that your Graph Editor curves are linear and not bezier (press T > Linear) so that it doesn't smooth the movement. Do the same for the next slippings.